In liquid-liquid extraction, the organic extraction solution is mixed into an aqueous solution, where said aqueous solution contains a dissolved is substance to be purified and concentrated, a metal often in ion form or as a complex together with numerous impurities. The valuable metal or element to be refined reacts selectively with an organic extraction chemical, whereby it is made to separate in pure form into the organic extraction solution. The aqueous solution depleted of the valuable metal or raffinate is generally routed back to ore or concentrate leaching. The valuable metal or element can then be separated from the organic solution by the reverse chemical reaction to extraction into a second aqueous solution (stripping). The organic extraction solution depleted of the valuable metal is recycled back to the extraction stage. The pure aqueous solution containing the valuable metal is processed further to recover the metal, e.g. by precipitation or reduction to metal (electrolytic recovery).
In some cases the aqueous solution exiting ore or concentrate leaching may contain several valuable metals. In this case the purpose of extraction may be to separate one or some of the metal impurities from the aqueous solution into an organic extraction solution, particularly if the amount of impure metal is considerable. Then the aqueous solution, removed from the extraction stage and purified of impure metals, is routed to further treatment, and the valuable impure metal is transferred from the extraction solution to another aqueous solution in stripping, and the extraction and aqueous solutions are processed as described above.
In accordance with the above, a liquid-liquid extraction system is formed of several stages, which are for example extraction proper, scrubbing of the organic solution, and stripping. Both the extraction and stripping stages may consist of several steps, whereby each step comprises an aqueous and organic solution mixer and a settler where the solutions separate from each other. The extraction solution scrubbing step is also formed of a mixer and settler or in some other appropriate way. An extraction stage is described in U.S. Pat. No. 5,662,871, where the mixer consists of a pumping tank, into which the aqueous solution and the extraction solution are fed, plus two mixers, in is which the mixture of solutions are further dispersed. The dispersion of the mixed solutions is routed to the settler unit of the extraction step to separate out into separate layers. It is characteristic of extraction that the organic solution used is recirculated in the extraction system. It is also typical of the system that there is a separate storage tank for the organic solution, which is equipped with pumping apparatus. In drawings the tanks are generally marked with the term LO tank, which means a tank for an organic solution containing valuable metals (Loaded Organic tank). Nowadays a storage tank is included in all liquid-liquid extraction systems as one unit. In some systems it may be what is termed a tank with scrubbing properties, but in addition the system also comprises a scrubbing step consisting of a mixer and settler.
The storage tank is in the system among other reasons so that part of the organic solution volume can be routed there in case of disturbance or production stoppage. The organic solution storage tank forms a large and expensive part of the system, since for instance in normal large extraction systems its volume may be of the order of 2000-2500 m3. The organic solution storage tank functions primarily as a pumping tank, which means that there has to be a certain solution depth in the tank in normal operating situations for pumping to succeed. Since the price of extraction solution is fairly high, the amount of extraction solution required in the storage tank for pumping raises the overall investment costs of the extraction system. Extraction solution is a flammable liquid, so the large quantity of organic solution circulating in a conventional extraction system increases the possible fire load of the liquid-liquid extraction process.